1. Field of the Invention
This invention relates generally to the field of insulating materials and more specifically to a method for manufacturing a fine porous member having a thermal conductivity which is far lower than that of stationary air under atmospheric pressure and which also is only slightly variable over time. The fine porous member of the invention can be effectively utilized in various applications as a heat insulator because of the low thermal conductivity.
2. Description of the Prior Art
Generally, heat insulators have a thermal conductivity in a range of about 0.03 to 0.05 kcal/mhr.degree.C. which is higher than the thermal conductivity of stationary air, namely, from 0.02 to 0.024 kcal/mhr.degree.C. In many applications the insulation provided by such insulators is insufficient, and attempts have been made to obtain a heat insulator having a thermal conductivity lower than that of stationary air.
In particular, in U.S. Pat. Nos. 3,869,334 and 4,564,547 to J. T. Hughes et al, there has been suggested heat insulators in which a fine porous member is utilized. More specifically, the former patent provides a technique of forming a panel of an integral insulator of a fine porous silica aerogel fixed with a glass fiber cloth under pressure, while the latter patent shows a heat-insulator manufacturing method according to which vacant space in a honeycomb structure of a metallic or organic material, woven textile, paper or the like is attempted to be reduced by charging such fine material as silica aerogel into the vacant space. With these heat insulators, it is possible to lower thermal conductivity to be about 0.02 kcal/mhr.degree.C. under atmospheric pressure so that the thermal conductivity can be remarkably improved as compared with general heat insulators.
The thermal conductivity of these known heat insulators under atmospheric pressure has been, however, still substantially at the level of the conductivity of stationary air and, accordingly, the heat insulators have been required to be made considerably lower in thermal conductivity under the atmospheric pressure than that of the stationary air, in order that the heat insulating effect can be well elevated. In this respect, the known heat insulators have been unable to attain a sufficiently satisfactory heat insulating effect. Further, the heat insulator adsorbs aerial vapor when left in the atmosphere, so that heat insulating properties deteriorate with time.
Further, there have been known heat insulators having low thermal conductivity such as a porous member of calcium silicate made in a vacuumed state of about 0.1 Torr, a foamed and crushed pearlite also in a vacuumed state of about 0.1 Torr as has been disclosed in Japanese Patent Laid-Open Publication No. 60-33479, and the like, but they have been defective due to the necessity of being kept in the vacuumed state, and due to higher manufacturing costs. In utilizing them as the heat insulator, they have to be considerably restricted in shape and usage because of the necessity of being kept in the vacuumed state, so as to be troublesome to be practically utilizable.
There has been also developed a heat insulator having a thermal conductivity of 0.015 kcal/mhr.degree.C. in the form of a rigid foamed urethane. In the case of this foamed urethane, however, the low thermal conductivity is solely dependent on the thermal conductivity of fluorocarbon gas (viz., 0.006 to 0.01 kcal/mhr.degree.C.) sealed in respective foams, so that there have been such problems that the heat insulating properties will be deteriorated once a substitution of air for the fluorocarbon gas takes place during long term use, the deterioration after one year reaching such an extent that the thermal conductivity is elevated to be about 0.021 to 0.024 kcal/mhr.degree.C.